Lithium hydroxystearate thickened grease having both stable short soap fibers and metastable soap microfibers and process for forming same



United States Patent 8 Claims. (Cl. 252-41) This invention relates to lubricating greases, and more particularly is concerned with providing a lithium soap thickened grease having mechanical stability and leakage characteristics superior to greases heretofore available.

' Lithium soap greases are gels comprising a major portion of normally liquid lubricating oil held within the interstices of a mat of twisted microscopic fibers of the lithium soap. According to conventionally accepted theories, fiber geometry (expressed as fiber length in microns, or as the ratio of fiber length to fiber width L/W) controls the physical characteristicsrof a grease;

Mechanical stability, the ability of a grease to resist hardening upon ageing and permanent softening after physical working, is normally associated with greases of relatively long fiber length. On the other hand, the ability of a grease to retain the lubricant oil in gel form, or leakage, depends on the size of spaces between soap fibers, and for greases of a given soap concentration, good leakage resistance is found with shorter fibers. From these considerations, it would seemingly be apparent that a grease having both good mechanical stability (favored by longer fibers) and also good leakage (imparted by short fibers) would be an inconsistent combination of properties unless excessive soap concentrations were employed. In fact, such patents as R. J. Moore U.S. 2,614,079, which employed a soap containing fine and coarse fibers reported that the mechanical stability and leakage of the grease were approximately intermediate between the properties of individual greases containing only one fiber length each.

- We have now discovered a method of preparing a lubricant grease in which both the desirable characteristics of goodmechanical stability and of good leakage are attained. We have discovered that a blend of two lithium hydroxystearate greases, one component containing short soap fibers which are in a stable crystalline phase, and the other .component containing microfibers in a metastable phase, has a mechanical stability which is markedly superior to that of either grease individually, yet retains the lubricant oil almost as well as a microfiber grease. In keeping with the nomenclature proposed by Fartington and Davis (I. Inst. Petroleum, 39, pp. 265-275, May 1953) the term short fibers" as employed in this paragraph and hereinafter in the specification and claims refers to soap fibers having an average length of between one and ten microns, while microfibers defines fiber lengths of less than one micron.

Thus, in accordance with the present invention, a first grease comprising a lubricant oil thickened with stable short soap fibers is mixed with a second grease comprising a lubricant oil thickened with metastable microfibers, and the mixture is milled. In a grease of the present invention, stable fibers may be advantageously formed by dissolving the lithium soap in a portion of lubricant oil and holding the resulting solution within the phase transition range of about 360-400 F. for at least about one hour and preferably between about two and three hours. Microfibers, when prepared by dissolving the lithium soap in oil,'heating'to'melt, and rapidly chilling the solution 1 through the transition range, exist in a crystalline phase which is different from the stable fibers. This phase difference is not apparent by microscopic inspection of the grease but may be demonstrated readily by means of differential thermal analysis techniques and confirms what would be predicted from accepted physical chemistry. Metastable microfibers have a particle size of less than one micron in length and a ratio of length to width of between about five and about fifteen; upon ageing over very long periods or upon heating in the transition temperature range, these will ultimately revert to fibers having the stable crystal form.

Soaps which are suitable for the present invention are the 9-, 10-, ll-, 12-, and 13-hydroxystearic acid soaps of lithium, of which the l2-hydroxystearic acid soap is most readily available and in fact is most preferred. These soaps may be prepared by reacting lithium hydroxide or lithium hydroxide monohydrate with either the correspending hydroxystearic acid or a glyceride thereof. For example, lithium 12-hydroxystearate is prepared either by reacting lithium hydroxide with free l2-hydroxystearic acid or with hydrogenated castor oil (hydrogenated glyceride of ricinoleic acid) preferably in the presence of a portion of the lubricant oil. The soap concentration in the final grease may be from about 3 to about 40% by weight, preferably from about 5 to about 15%, and when in accordance with the present invention two lithium hydroxystearate thickened'greases are blended, the same hydroxystearateor a mixture of different hydroxystearates may be used. The soap concentrations in each individual grease prior to blending are preferably approximately (i.e., or the same. As available commercially, a hydroxystearic acid or its glyceride is usually only about -90% pure, with the remaining material being stearic acid and/ or other hydroxystearic acids. 1

Mineral oils, particularly those characterized in having highly branched isoparaffinic structures are the preferred lubricants for the greases of the invention. Mineral oils having a viscosity offrom about 55 to 2000 SSU seconds at 100 F. and preferably about 300 SSU seconds are especially suitable. In lieu of mineral based oils, synthetic lubricant oils such as the aliphatic diesters typified by di-Z-ethyl hexyl sebacate, the aromatic diesters such as di-2-ethyl hexyl phthalate, the polymerized ethylene oxides, the siloxanes, or the fluorocarbons may be used.

7 In general, it is best to employ the same class of oils in a blended grease, even though the compositions may be slightly different. For example, two dilferent ester oils may be used, but one mineral oil based grease and one containing a siloxane raise compatibility questions.

For the preparation of a grease containing stable short fibers, a preferred method involves adding a stoichiometric amount of lithium hydroxide monohydrate, preferably in a water slurry, to hydrogenated castor oileand about one-third of the total quantity of mineral oil in a grease kettle. Upon commencing heating, lithium hydroxide and hydrogenated castor oil react to form the lubricant base, and the temperature within the kettle is gradually raised to about 250 P. so as to drive off the resulting water. The base is then heated to about SSS-425 F., preferably above 400 F., to assure that all of the soap melts and dissolves in the oil phase. Complete solution is indicated by the oil becoming clear and translucent. The temperature is then slowly reduced to within the transistion range of about 360-400" F. where stable grease fibers form, and is maintained in this range-either at a constant temperature or with slow cooling-for at least an hour and preferably two or three hours or more. After the hour or so elapses, grease fibers of desired length are formed and the remaining two-thirds of the oil is slowly graded in, again with slow cooling. At a temperature'of about 200-275" F.,-thegrease is milled in a conventional colloid mill or homogenizer to disperse the soap fibers. By the above technique, stable soap fibers having an average L/W of between about 17 and 26, and a fiber length of between about one and ten microns are produced and dispersed.

Greases thickened with metastable microfibers are preferably manufactured by reacting an aqueous slurry of lithium hydroxide monohydrate with the stoichiometric requirement of free l2-hydroxystearic acid (about 87% pure, the balance being predominantly stearic acid) in the presence of about one-third of the necessary oil. The base is heated to about 250 F. while saponification' is taking place and to drive off water, continuing heating, adding the balance of the oil at about 300 F., and then heating further to above about 385 F. (preferably above about 400 F.) so as to dissolve all of the soap in the oil phase. To obtain metastable fibers and avoid formation of soap fibers having the stable crystalline form, the mixture is shock-chilled within a few seconds through the 360-400 F. transition range to a temperature preferably within the range of 100-150 F. Shock-chilling is accomplished by charging the melted grease to a pan beneath a water-cooled rotating drum; as the drum rotates a thick film of grease is deposited thereon and is instantly chilled through the transition range. blade and milled at 200 F. or below.

The short fiber grease and the microfiber grease are blended by charging the two greases, preferably in a ratio of between 1:3 and 3:1 on a weight basis, to a conventionally heated grease kettle wherein the greases are heated to about 150-200 F. with slow stirring for about a half hour or more. After this blending, the soap fibers are intimately dispersed by milling the grease in a colloid mill or homogenizer, preferably the former.

Several individual greases of diiferent soap concentrations were prepared according to the above techniques, and the individual greases and blends thereof were inspected by the following tests:

(1) Length to width razio.-A grease smear is photographed at 3200 diameters magnification under the electron microscope and a 10 power enlargement made of the photograph. At least 500 individual grease particles are measured for their length and width, and the ratio of arithmetical average length to average width is computed for each grease.

(2) PeneZratin-ASTM D217-52T.A measure of grease hardness or consistency is obtained by measuring the distance a falling weighted cone enters a cup of grease.

(3) Mechanical stability.-The resistance of a grease to change upon physical working is measured by determining the ASTM penetration on a grease as obtained from the mill or homogenizer, again after working 60 strokes in the ASTM D2l7-52T Worker, and again after working 100,000 strokes. The percentage change in penetration is taken as a quantitative indication of the mechanical stability.

(4) Roll stability.-The mechanical stability of a grease while working under compressive load is determined by rolling a simple of the grease in a tube containing an eleven pound bar, in accordance with the test described in Inst. Spokesman, 6 (1943), page 2. The change in ASTM penetration before and after rolling is a measure of mechanical stability in rolling friction service under load.

(5) Leakage, 0ven.A gram grease sample in a G'O-mesh screen cone is placed on a tared beaker in an oven at 212 F. Leakage is determined by weighing the amount of oil in the beaker after 24 hours testing.

(6) Leakage, RIA--Federal Standards 79], Method 322-T.-The Rock Island Arsenal pressure bleeding test comprises weighing the leakage from a /2 inch layer of grease on a 100 mesh screen after subjecting the grease to 0.35 p.s.i.g. air pressure for 24hoursat 77 F.

The grease is scraped off the drum with a doctor EXAMPLE I A grease blend was prepared from two mineral oil based greases, one thickened with eight percent lithium 12-hydroxystearate soap as stable shoft fibers, and the other, in an equal amount, of a grease thickened with eight percent of the same soap but in metastable microfiber form. The mineral oil for each individual'grease Was a mixture of 13 volume percent solvent-extracted SAE-4O oil and 87 of low cold test Winkler (Texas) lubricating oil. The mixture of oils had a viscosity of 110.2 SSU seconds at 100 F. and 39.1 seconds at210 F., a viscosity index of 47, an API gravity of 25.1, a 330 F. flash point and a pour of -60 F. The two individual greases, each after one pass through a conventional colloid mill, were inspected by the six tests shown above; the blended grease was mixed at 150 F., colloid milled once, and similarly inspected. The results of these tests are presented in Table I below.

Table I Microfiber Short -50 Grease Fibers (Mcta- Fiber Blend stable) Soap, wt. percent 8. 0 8.0 8.0 No. Millings l 1 l L W 8. 4 22.8 12. 1 Penetrations:

Unworkei 267 302 282 60 Strokes 272 299 273 100,000 Stro 337 348 321 Roll Stability, 77 F Before 106 113 112 After 130 128 115 Percent Change 31. 2 13.3 2. 7 Leakage:

It will be observed from the above that the 50-50 mixture according to the instant invention exhibits remarkably improved mechanical stability, both under the conditions of ASTM Working and in weighted roll stability, as compared with either grease individually. Its change in penetration after working 60 strokes and after working 100,000 strokes is substantially less than that of either the microfiberor the short fiber grease, and in th'eroll stability, the penetration change is only about one-fifth that of a short fiber grease which is usually considered to possess good mechanical stability. Moreover, the improved blended grease has less leakage than even the microfiber grease (which usually has exceptional leakage properties) under the conditions of the oven test, and is intermediate between the two individual greases in the RIA leakage test.

EXAMPLE II Table II Grease Fibers Microfiber Short 50-50 (Meta- Fiber Blend stable) Soap, wt. Percent 7.0 7.0 7.0 No. Millings 2 2 2 L/W 12. s 22. 1 17. 3 Penetrations:

Unworked 296 301 295 60 Strokes.-. 296 295 284 100,000 Strokes 378 342 328 Roll Stability, 77 F.:

Before 265 282 263 After i .861 .337 .I 297. Percent Change- 35. 8 l9. 5 12. 9 Leakage: ven 2. 53. 3.9 3.341 RIA 3.00 10. 65 4. 45

From the above Table II it is quite evident that the blended grease of the present invention has mechanical stability superior to either the microfiber or the short fiber greases from which it is made. Under the conditions of the ASTM worker and in the roll stability test, the improved grease exhibits mechanical stability well in excess of that displayed by either the microfiber or the short fiber grease. Also, it will again be observed that the leakage characteristics of the blended grease are closer to those of the microfiber grease.

From the foregoing studies of the preceding examples, it is seen that a grease prepared according to the present invention and containing short soap fibers and metastable microfibers is superior with respect to mechanical stability to a grease containing either fibers alone, and exhibits leakage which is superior to short fiber greases and almost as favorable as microfiber greases. Thus it is possible, with conventional grease making equipment, to manufacture a superior lithium hydroxystearate thickened grease which has characteristics heretofore believed unobtainable in a single grease. By separately preparing a microfiber grease and a short fiber grease, blending the two, and milling the blend, a grease is obtained which will satisfy the most severe service requirements. If desired, conventional grease additives such as EP agents, anti-rusts, antix-oxidants, oiliness agents, etc. may be incorporated in the blend during processing.

. Thus having described the invention, what is claimed is:

1. A process for preparing a blended lithium hydroxystearate thickened grease having both stable short soap fibers having a fiber length between about 1 and l microns and an average ratio of fiber length to fiber width between about 17 and 26 and metastable soap microfibers having a fiber length of less than 1 micron and a ratio of fiber length to fiber width of between about 5 and 15, said grease being characterized in having mechanical stability superior to individual greases containing either fibers alone, which process comprises mixing a first grease comprising a lubricating oil thickened with stable short lithium hydroxystearate fibers, with a second grease comprising a lubricating oil thickened with metastable lithium hydroxystearate microfibers said short fiber grease and said microfiber grease being present in a ratio of between 1:3 and 3:1 respectively, and milling the blend.

' 2. Process of claim 1 in which the lithium hydroxystearate is predominantly lithium 12-hydroxystearate.

3. A blended lithium hydroxystearate grease composition being characterized in having improved mechanical stability and leakage characteristics prepared by the method of claim 1.

4. A process for preparing a lithium hydroxystearate grease composition having improved mechanical stability and leakage characteristics comprising blending together (a) a stable grease of short fibered soap having a fiber length between about 1 and 10 microns and an average ratio of fiber length to fiber width between about 17 and 26, prepared by heating a mixture of a mineral lubricating oil and lithium l2-hydroxystearate to atleast the soap solution temperature, holding the mixture in the transition range of about 360-400 F. for at least about one hour, then slowly cooling the mixture to a temperature of about 200275 F., and milling and (b) a metastable grease of microfibered soap having a fiber length of less than 1 micron and a ratio of fiber length to fiber width of between about 5 and 15 prepared by heating a mixture of a mineral lubricating oil and lithium 12-hydroxystearate to at least the soap solution temperature, shock chilling to a temperature within the range of 100-150" F., and milling, and heating the blended stable short fiber soap grease and metastable microfiber soap grease to a temperature of about 150-200 F. with slow stirring and milling the heated blended greases.

5. Process of claim 4 in which the lubricant oil is a mineral oil having a viscosity of from about to 2000 SSU seconds at F.

6. A blended lithium l2-hydroxystearate grease composition having improved mechanical stability and leakage characteristics prepared by the method of claim 4.

7. The blended lithium' 12-hydroxystearate grease composition of claim 6 in which the lithium l2-hydroxystearate soap is present in amounts of from about 4% to about 15% by weight.

8. The blended lithium 12-hydroxystearate grease UNITED STATES PATENTS 2,588,556 Moore et al Mar. 11, 1952 2,614,079 Moore Oct. 14, 1952 2,846,394 Brunstrum et a1. Aug. 5, 1958 Zweifel et a1. Aug. 4, 1959 

1. A PROCESS FOR PREPARING A BLENDED LITHIUM HYDROXYSTEARATE THICKENED GREASE HAVING BOTH STABLE SHORT SOAP FIBERS HAVING A FIBER LENGTH BETWEEN ABOUT 1 AND 10 MICRONS AND AN AVERAGE RATIO OF FIBER LENGTH TO FIBER WIDTH BETWEEN ABOUT 17 AND 26 AND METASTABLE SOAP MICROFIBERS HAVING A FIBER LENGTH OF LESS THAN 1 MICRON AND A RATIO OF FIBER LENGTH OF LESS THAN 1 MICRON ABOUT 5 AND 15, SAID GREASE BEING CHARACTERIZED IN HAVING MECHANICAL STABILITY SUPERIOR TO INDIVIDUAL GREASES CONTAINING EITHER FIBERS ALONE, WHICH PROCESS COMPRISES MIXING A FIRST GREASE COMPRISING A LUBRICATING OIL THICKENED WITH STABLE SHORT LITHIUM HYDROXYSTEARATE FIBERS, WITH A SECOND GREASE COMPRISING A LUBRICATING OIL THICKENED WITH METASTABLE LITHIUM HYDROXYSTEARATE MICROFIBERS SAID SHORT FIBER GREASE AND SAID MICROFIBER GREASE BEING PRESENT IN A RATIO OF BETWEEN 1:3 AND 3:1 RESPECTIVELY, AND MILLING THE BLEND. 